(1) Field of the Invention
The present invention is related to complementary metal oxide semiconductor (CMOS) photodiodes and photogates, more specifically, the method and apparatus of the present invention is related to a focal plane averaging implementation for CMOS imaging arrays.
(2) Background
The capture of image data using CMOS imaging devices is typically achieved with arrays of picture sensing elements or pixels, where each pixel consists of a photodiode or a photogate, a charge storage capacitance, exposure time control elements and read out devices. A photodiode (or a photogate) is a semiconductor device that produces significant photocurrent when illuminated. A photocurrent is an electric current produced in a device by the effect of incident electromagnetic radiation in the visual frequencies. An image focused on an array of pixels excites the photosensors within pixels of the array differently resulting in different charge and corresponding signal values directly related to the quantum of energy incident upon the pixels.
The registration of images through a semiconductor imaging array is associated with numerous noise components, such as reset noise, read noise, shot noise and fixed pattern noise, many of which are eliminated by known architectural and circuit techniques, such as correlated double sampling and delta double sampling. Noise may also be eliminated by using system level solutions such as dark frame subtraction.
Noise includes undesired electrical signals that occur within electronic or electrical devices, circuits, or apparatus and that result in changes in the values obtained. Noise components in electronic image capture which are not eliminated despite the implementation of the techniques mentioned, include flicker or 1/f noise in an output chain, thermal (kT/C) noise, A/D (analog-to-digital) quantization noise and photon/dark shot noise.
Flicker noise, also known as low frequency or excess noise, is a noise component with a spectral density which increases as frequency decreases, and is in excess of the thermal and the shot noise components within semiconductor components. Flicker noise has a Gaussian Probability Density Function (PDF) and its spectral density is known to be proportional to the electrical current passing through the device in consideration and inversely proportional to the frequency.
Thermal noise, also known as Johnson or Nyquist noise, induces a fluctuating potential difference across the ends of any conductor that is caused by the random, thermally induced motion of electrons within the conductor. The spectral density of this noise component, which also has a Gaussian PDF as anticipated for a phenomenon that results from a large number of random events, is proportional to the electrical resistance and the bandwidth of the conductor.
Shot noise in semiconductor devices results from the passage of charged carriers across a semiconductor junction associated with a potential barrier. Shot noise, by definition, is noise caused by the random passage of carriers across a potential barrier and the spectral density of the shot noise is directly proportional to the average electrical current flowing through the potential barrier. Shot noise exhibits itself as a fluctuating emf whose mean square value is proportional to the average current flow.
All noise sources in a CMOS imaging array results in a significant pixel to pixel variation in the value registered when the imaging array is exposed to a uniform field of elimination. At very low light levels dark shot noise is seen to be the dominant noise component that results in a grainy resultant image, particularly in imaging arrays where dark current is a significant value within the pixels. Dark shot noise is the random component associated with dark current, which is the leakage current from the pixel storage node in the dark.
Other noise components not eliminated by traditional architectural techniques act incrementally to reduce the signal to noise ratio (SNR) in low light imaging using CMOS sensors. Examples of effects that degrade SNR and are not removed despite minimization architectures include reset noise and photo response non-uniformity on a photosensor layer. More specifically, reset noise is a random variation in the xe2x80x9cRESETxe2x80x9d value within a pixel that is not removed due to the implementation of a time uncorrelated reset operation between the full array reset and the readout reset phases. Photo response non-uniformity is the non-uniformity in the response of the pixels of the imaging array to a uniform field of illumination and is a consequence of processing/manufacturing imperfections.
A typical photodiode-based pixel layout on a standard CMOS process has the advantage that space within the pixel is optimally utilized. However, the structure has disadvantages in that each pixel""s photodiode is placed right next to circuitry within adjacent pixels and hence is susceptible to cross-talk, which could be signal dependent, as well as of a fixed nature. The photodiodes are also separated from each other by the pixel pitch and are therefore susceptible in terms of adjacent pixel variability in dark/photo response. The photodiodes are also susceptible to semi-conductor processing related variations, which could have spatial frequencies comparable to the pixel pitch.
It is desirable to have a method and apparatus on CMOS photodiode/photogate based imaging integrated circuits with multiple metal layers for achieving focal plane averaging, which has associated advantages in the signal to noise ratio (SNR).
A photosensor structure including pixel areas. Each pixel area represents an electrical pixel. The photosensor structure also has a predetermined number of quadrants. Each is positioned in a corner of one of the pixel areas. Each quadrant belongs to different optical pixels.